This invention relates to an aerial lift apparatus having a boom assembly wherein the boom assembly comprises at least two boom members and wherein one of the boom members is secured to a mobile base and is adapted, at least, for vertically pivotable movement about a first horizontal axis with respect to the mobile base and wherin the second boom member is carried by the first boom member. A third boom member, if any, can be carried by the second boom member and a fourth boom member, if any, can be carried by the third boom member, and so on. Each of the boom members in such a lift apparatus is typically adapted to move longitudinally with respect to the boom member on which it is carried, usually by means of an extension means, such as a hydraulic cylinder or the like. However, the present invention is not limited to boom assemblies that have such separate extension means to reciprocate one boom relative to the other.
The lift apparatus of the type contemplated by this invention may be a tubular telescoping assembly having a work platform or basket on the distal end of the last, or outermost, boom member. Such a platform or basket is typically used for supporting workmen or firemen. Alternatively, the present invention may be incorporated in an assembly of ladders which are slidably disposed, one upon the other, and adapted to extend and retract, in a quasi-telescoping manner. Such a ladder assembly would typically be used on mobile firefighting trucks.
There are presently several types of telescoping and quasi-telescoping basket-carrying booms or ladders on the market. One such ladder assembly, in combination with a pumper fire truck, is disclosed in the U.S. Pat. No. 3,675,721 to Davidson et al. An extensible assembly of ladders, in combination with a truck, which assembly of ladders has a personnel-carrying basket on the distal end, is disclosed in the U.S. Pat. No. 3,572,467 to Hall.
Boom assemblies of the type discussed above are typically arranged to be carried in a retracted, generally horizontal position, usually on the top of a mobile support truck, and are pivotally mounted about a horizontal axis so that the assembly may be rotated from the horizontal position through an increasing included vertical angle with respect to the earth's surface. Most of these types of apparatus are intended to be used over a wide range of vertical angle orientations with respect to the earth's surface. Specifically, some, though not all, may be used in a substantially vertical position as well as in a horizontal, or even slightly below horizontal position.
The types of lift apparatus disclosed in the above-described patents are usually also adapted to be rotated about a vertical axis on the mobile support base or truck whereby the extended, distal end of the ladder (and personnel-carrying basket or platform, if any) may be moved in a circular path in a plane perpendicular to the vertical axis (and hence, in a circle which is generally parallel to a plane tangent to the earth's surface at that pont).
It is well known to those skilled in the art to which this invention pertains that, when the boom assembly of the lift apparatus of the type described is in a position other than in the true vertical, there is a moment created by the cantilever effect of the boom assembly which is overhanging the support base. This is especially significant when such a boom assembly is carried by a truck and when the boom assembly is rotated about a vertical axis away from alignment with the length of the truck so that the boom assembly extends outwardly to one side of the truck. The tipping moment, of force tending to overturn the support truck, as well as the structural moments imposed upon the various members, depend upon the length of the boom (in the fully extended position) and upon the weight of the boom assembly (including the weight of any platform and/or personnel at the outer end of the boom assembly).
Typically, for a given maximum design extension length and for a given boom weight (including the weight of any platform and live load associated therewith), the boom assembly base support must be appropriately designed to withstand the overturning moment. With a mobile truck support, separate retractable outriggers or struts may be provided on the truck body for extending outwardly and downwardly to contact the ground on either side of the truck to thereby increase the effective width of the support and to thereby counteract the overturning moment. Nevertheless, for a given truck and retractable strut or outrigger design, there is a maximum extended length associated with any given boom design which can be accommodated on the truck in any horizontal position without overturning the truck or exceeding allowable structural moments.
It is well known to those skilled in the art to which the present invention pertains that, as the boom assembly is lifted upwardly from a horizontal position with respect to the earth's surface, the overturning moment decreases because the effective moment arm of the boom assembly decreases. Thus, when moving a fully extended boom from the horizontal position to some elevated position, the actual height to which the boom could be further extended, without overturning the truck, increases. For example, for a given boom assembly design, a boom could have a maximum horizontal extension of, say, 76 feet at the point just below which the support truck would tend to overturn. However, in the elevated position, say at a vertical angle of 85 degrees with respect to the earth's surface, the boom could be extended an additional number of feet before the support truck would tip. If it were possible to somehow gain this "extra extension" as the boom assembly was rotated upwardly in an increasing vertical angle with respect to the earth's surface, there would be the great advantage of being able to obtain a greater reach height.
Heretofore, prior art lift apparatus of the general type to which this invention pertains, have had to take such matters into consideration and have adopted a variety of solutions, such as decreasing the boom length, decreasing the boom weight, using counterweights opposite the boom extension, and sacrificing the maximum reach height associated with a given vertical angle in order to prevent the mobile support assembly from being overturned when the boom assembly was at its fully extended, horizontal position.
The inventor of the present invention knows of no system where the boom assembly is automatically extended with an increasing included vertical angle as simply as in accordance with the present invention. It has been found to be highly desirable to provide, in a lift apparatus of the type described, means extending the boom assembly automatically and by direct mechanical linkage, as the boom assembly is rotated in an increasing included vertical angle with respect to the earth's surface so as to take advantage of the maximum reach height possible without exceeding the allowable tipping and structural moments.